The present invention relates to catalysts useful to reduce/hydrogenate ketones, aldehydes and imines. More specifically, it relates to improved ligand iron metal bifunctional catalysts for that purpose.
In connection with a variety of organic syntheses it is desirable to hydrogenate particular functional groups. Particularly desirable reactions are those which are efficient, selective insofar as hydrogenating only those groups of interest, produce few by-products that complicate purification, and use only relatively inexpensive materials.
Hydrogenation reactions often involve use of a chemical such as sodium borohydride or lithium aluminum hydride. However, these are very powerful reducing agents, making them not particularly selective.
A variety of enzymatic “hydrogenase” catalysts have been developed that facilitate hydrogenation reactions of various types. See generally J. Peters et al., X-ray Crystal Structure Of The Fe-Only Hydrogenase (Cp1) From Clostridium pasteurian To 1.8 Angstrom Resolution, 282 1853 et seq. (1998); Y. Nicolet et al., Desulfovibrio Desulfuricans Iron Hydrogenase: The Structure Shows Unusual Coordination To An Active Site Fe Binuclear Center, 7 Structure 13-23 (1999); H.-J. Fan et al., A Capable Bridging Ligand For Fe-Only Hydrogenase: Density Functional Calculations Of A Low-Energy Route For Electrolytic Cleavage And Formation Of Dihydrogen, 123 J. Am. Chem. Soc. 3828-3829 (2001). However, enzymatic catalysts often cause a variety of purification and other concerns, and often require a narrow range of reaction conditions which may not be optimal for other reasons during a synthesis.
There have been attempts to try Ru or Rh based bifunctional catalysts for the hydrogenation of polar multiple bonds. See R. Noyori et al., Asymmetric Catalysis By Architectural And Functional Molecular Engineering: Practical Chemo- And Stereoselective Hydrogenation Of Ketones, 40 Angew. Chem. Int. Ed. 40-73 (2001); T. Ikariya et al., Bifunctional Transition Melta-Based Molecular Catalysts For Asymmetric Syntheses, 4 Org. Biomol. Chem. 393-406 (2006); Y. Shvo et al., A New Group Of Ruthenium Complexes: Structure And Catalysis, 108 J. Am. Chem. Soc. 7400-7402 (1986); C. Casey et al., Hydrogen Transfer To Carbonyls And Imines From A Hydroxycyclopentadienyl Ruthenium Hydride: Evidence For Concerted Hydride And Proton Transfer, 123 J. Am. Chem. Soc. 1090-1100 (2001).
See also J. Casey et al., Isomerization And Deuterium Scrambling Evidence For A Change In the Rate-Limiting Step During Imine Hydrogenation By Shvo's Hydroxycyclopentadienyl Ruthenium Hydride, 127 J. Am. Chem. Soc. 1883-1894 (2005); J. Casey et al., Reduction Of Imines By Hydroxycyclopentadienyl Ruthenium Hydride: Intramolecular Trapping Evidence For Hydride And Proton Transfer Outside The Coordination Sphere Of The Metal, 127 J. Am. Chem. Soc. 14062-14071 (2005); J. Casey et al., Stereochemistry Of Imine Reduction By A Hydroxycyclopentadienyl Ruthenium Hydride, 128 J. Am. Chem. Soc. 2286-2293 (2006). However, Ru and Rh are relatively expensive to obtain and use.
There have also been a number of attempts to use certain iron-based catalysts in connection with certain hydrogenation reactions. Some of these catalysts selectively catalyze alkene hydrogenation. See M. Schroeder et al., Pentacarbonyliron(O) Photocatalyzed Hydrogenation And Isomerization Of Olefins, 98 J. Am. Chem. Soc. 551-558 (1976) (Fe(CO)5; S. Bart et al., Preparation and Molecular and Electronic Structures of Iron(O) Dinitrogen And Silane Complexes And Their Application To Catalytic Hydrogenation And Hydrosilation, 126 J. Am. Chem. Soc. 13794-13807 (2004); E. Daida et al., Considering FeII/IV Redox Processes As Mechanistically Relevant To The Catalytic Hydrogenation Of Olefins by [PhBP/PR3] Fe—Hx Species, 43 J. C. Inorg. Chem. 7474-7485 (2004). See also M. Radhi et al., Hydrogenation Of N-Benzylideneaniline With Molecular Hydrogen Using Iron Pentacarbonyl As Catalyst Precursor, 262 J. Organomet. Chem. 359-364 (1984). However, these publications did not report similar success in hydrogenating certain other groups such as ketones and aldehydes.
Also, there was a disclosure by Steven W. Singer, on pages 232-239 of an appendix of his PhD thesis entitled “Formation And Reactions Of A Hydroxycyclopentadienyl Ruthenium Hydride: An Organometallic Complex Containing Electronically Coupled Acidic And Hydridic Hydrogens”, University Of Wisconsin (2002), regarding synthesis of a ditolyl, diphenyl, hydroxycyclopentadienyl iron hydride compound, and its use in transferring hydrogen to an aldehyde. However, the catalytic capabilities of this compound were marginal, and there were other issues of concern regarding its use.
In unrelated work, in H.-J. Knoelker et al., Demetalation Of Tricarbonyl(cyclopentadieone)iron Complexes Initiated By A Ligand Exchange Reaction With NaOH-X-Ray Analysis Of A Complex With Nearly Square-Planar Coordinated Sodium, 38 Angew. Chem. Int. Ed. 2064-2066 (1999), the authors reported on the isolation of an intermediate having the following formula, where “TMS” refers to trimethyl silyl:

However, that article did not propose a catalytic utility for this compound, much less suggest any potential hydrogenation-related function.
Thus, there is a continuing need for improved catalysts for use in hydrogenation reactions.